The growth and maintenance of the skeleton is a dynamic process in which osteoblast cells form new bone and osteoclasts degrade bone. Src gene disruption experiments in mice demonstrated that Src, an intracellular protein tyrosine kinase, is essential for resorption activity of osteoclasts. Osteoporosis is a serious disease of essential bone degradation. The properties of Src deficient osteoclasts suggest that inhibitors of Src function would be useful drugs for the prevention or treatment of osteoporosis, by slowing down the rate of osteoclast mediated bone resorption. The investigators aim to exploit their knowledge of Src structure and function to develop orally active small molecule drugs that block Src function in osteoclasts. Src becomes activated in response to a variety of extracellular signals. The protein is composed of multiple structural domains, and among these, the SH2 and SH3 domains are critical for Src function. These non- catalytic domains promote protein-protein interactions between Src and key cellular proteins. The aim of the proposal is to 1) identify peptide ligands for Src SH2 and SH3 domains that bind with high affinity and specificity, 2) evaluate the peptides in vitro and in cultured cells for their ability to inhibit known Src functions, and 3) evaluate the peptides in cultured osteoclasts for their effect on Src-mediated signal transduction and resorption, and 4) determine the structures of domain- peptide complexes. These studies test the concept that a Src inhibitor will block osteoclast function and provide the foundation for structure- based drug development.